Process for electrolytically preparing uranium metal

ABSTRACT

A process for making uranium metal from uranium oxide by first fluorinating uranium oxide to form uranium tetrafluoride and next electrolytically reducing the uranium tetrafluoride with a carbon anode to form uranium metal and CF 4 . The CF 4  is reused in the fluorination reaction rather than being disposed of as a hazardous waste.

The invention was developed pursuant to a contract with the U.S.Department of Energy.

This invention relates to a process for making uranium metal fromuranium oxide using a fluorination by CF₄ and then by electrolyticallyconverting UF₄ to U. Since CF₄ is the fluorinating agent for making theUF₄ as well as the by-product of the electrolysis it is regenerated andreused in the process.

BACKGROUND OF THE INVENTION

Electrolytic reduction has been studied for the preparation of uraniummetal from uranium oxides. The desired overall reaction is UO₂ +2C→2CO+U, and the process has many similarities to the Hall-Heroultprocesses for preparation of aluminum from aluminum oxide using moltencryolite. At temperatures above the melting point of uranium, uraniummetal product of good purity was achieved but yields and currentefficiencies were very low. Satisfactory feed of UO₂ to electrolyticcells is much more difficult than the feed of Al₂ O₃ to Hall cells. Thesolubility of UO₂ is about one-tenth that of Al₂ O₃ in Hall cells andthe dense UO₂ settles to the bottom where it fouls the metal surface.

Yields and current efficiencies were improved by using compartmentalcells to confine the oxide feed and prevent fouling of the metalsurfaces. The current efficiencies were 31% as a maximum, and the celldesign was not suited to scale-up to large size.

An alternative reduction reaction, UF₄ +C→CF₄ +U, is possible. However,this reaction requires a makeup of UF₄ which is made from the reactionof UO₂ and large amounts of HF, a very costly chemical. Furthermore, CF₄is a more troublesome gaseous waste than is CO.

Preparation of uranium in electrolytic cells has been successfullydemonstrated. One study showed good cell operation using UF₆ as thefeed. In another case, a large electrolytic cell was operated with 30 to50% of the U metal from reduction of UO₂, 50 to 70% from reduction ofUF₄. The objective of these studies was to maximize the amount of UO₂feed; the higher the percentages of UF₄ were easier to use, but a largefraction of UF₄ feed with little UO₂ was not tested. A problem revealedby these studies was that UO₂ settled out in the electrolysis cell dueto its low solubility and high density.

SUMMARY OF THE INVENTION

In view of the above needs, it is an object of this invention to providea process for electrolytically preparing uranium metal from UF₄ with arecycling of the fluoride.

It is another object of this invention to provide a process forelectrolytically preparing uranium metal wherein fouling of theelectrolytic cell is minimized.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

To achieve the foregoing and other objects and in accordance with thepurpose of the present invention, as embodied and broadly describedherein, the process of this invention may comprise: a first reactionwherein a fluoride and UO₂ are reacted to form UF₄ ; a second reactionwherein the UF₄ formed in the first reaction is electrolyticallyconverted to U.sup.° by using a carbon anode resulting in the formationof CF₄ as by-product., and subsequent to the second reaction, the CF₄formed is recycled into the first reaction for reuse as fluorinatingagent. The process allows the recycling of a troublesome fluorocarbon,CF₄, and also minimizes the feed of UO₂ to the electrolytic cell thusavoiding the problem of fouling the metal surface. Also the consumptionof the carbon anode is reduced since the waste product is CO₂ instead ofCO.

FIGURE

The FIGURE is a schematic chemical flowsheet representing theelectrolytic reduction process of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is acombination of an electrolytic cell where the principal overall reactionis UF₄ +C→CF₄ +U.sup.° with a feed reactor where the primary reaction isCF₄ +UO₂ →UF₄ +CO₂. The two reactions together give the preferredoverall reaction UO₂ +C→CO₂ +U.sup.°. Moderate amounts of UO₂ thatremain in the UF₄ feed to the cell will dissolve and produce metal bythe overall reaction UO₂ +2C→2CO+U.sup.°. The utilization of twoseparate reactors allows separate optimization of the operatingconditions of each reaction. The electrolytic cell is optimized forreduction to metal without concern about the amounts of CO and CF₄products. The feed rector is optimized for recycle of fluorides as UF₄to give an efficient conversion of reactants to UF₄, thus eliminatingthe mixing and settling of UO₂ solids as a cell operating problem. Theconsumption of anode carbon is reduced because the final by-product ismostly CO₂ instead of CO.

The reactions of CF₄ with uranium oxides are thermodynamicallyfavorable. A number of reactions can be written with UO₂ and CF₄ asreactants and UF₄ as one product. Some of interest are:

    UO.sub.2 (s)+CF.sub.4 (g)→UF.sub.4 (s)+CO.sub.2 (g)

    UO.sub.2 (s)+CF.sub.4 (g)+H.sub.2 (g)→UF.sub.4 (s)+CO(g)+H.sub.2 O(g)

Both these reactions are favored with Σ∇_(f) G.sup.° of -310. KJ mol⁻¹and equilibrium constants at 1100° K. of about 5×10¹⁴. This resultagrees with the behavior in-cell where UF₄ reacts to form CF₄ only whenthere is a deficiency of UO₂. The second reaction is probably morefavorable as an excess of H₂ and the two gaseous products should allow amore complete utilization of the CF₄. The H₂ can also provide a completereduction of U(VI) to U(IV).

The modified electrolytic reduction process can be shown as a schematicchemical flowsheet as shown in the FIGURE. Fluorination 1 takes place inthe fluorination and reduction reactor where UF₄ is produced to be sentto the electrolytic cell accompanied by a small amount of UO₂. Alsoproduced in the fluorination and reduction reactor are byproducts CO₂,H₂ O, H₂, CO, HF and a small amount of CF₄ that are expelled as offgases5. The electrolytic reaction 3 takes place in the cell containing acarbon anode and produces uranium metal product as well as CF₄by-product that is recycled to the fluorination reaction 1 along with asmall amount of CO.

In prior studies of electrolytic reduction, the use of UO₂ wasconsidered preferable to the use of UF₄ to avoid evolution of theundesirable CF₄ by-product. Therefore, even when UO₂ and UF₄ were usedtogether as starting materials, which improved efficiency, the use ofUF₄ was minimized to minimize CF₄ offgas. This invention solves thatproblem by reusing instead of disposing of waste CF₄.

I claim:
 1. A process for converting UO₂ to uranium metal comprising:afirst reaction wherein a fluoride compound and UO₂ are reacted in afirst reaction chamber to form UF₄ ; a second reaction wherein said UF₄and unreacted UO₂ are electrolytically converted in a second reactionchamber, said second reaction chamber being an electrolytic cell, to U°by using a carbon anode resulting in the formation of CF₄ as by-product;and subsequent to said second reaction, recycling said CF₄ into saidfirst reaction for reuse of said fluoride.
 2. The process of claim 1wherein said first reaction further comprises adding H₂ in said firstreaction chamber.